Device and method for reducing slip in the control system of a CVT in a motor vehicle

ABSTRACT

The invention proceeds from a control of a transmission which is adjustable continuously with respect to its transmission ratio, for a motor vehicle. The transmission together with a drive unit is mounted in the drive train of the motor vehicle and the drive unit has an adjustable drive torque. The transmission includes a drive end as well as an output end and operative means for establishing a mechanical operative connection between the drive end and the output end. Furthermore, detection means for detecting a slip quantity is provided. The slip quantity represents the slip between the operating means and the drive end and/or output end. Pregivable measures are initiated in response to a pregiven value of the slip quantity. The essence of the invention is that measures as follows are initiated: a drive of a clutch mounted in the drive train; and/or, a change of the transmission ratio; and/or, a change of the output torque of the drive unit. It is especially intended that measures are initiated in the sense of a slip reduction.

FIELD OF THE INVENTION

The invention relates to an arrangement and a method for controlling atransmission in a motor vehicle. The transmission is continuouslyadjustable with respect, to its transmission ratio.

BACKGROUND OF THE INVENTION

In a motor vehicles having a known continuously variable transmission(CVT) with a continuous element (for example, a thrust element belt orchain), the contact engaging force of the conical pulleys against thethrust element belt must be so adjusted via suitable measures that thetorque, which is to be transmitted, can be transmitted. A high slippagebetween the continuous element and the pulleys occurs when the contactengaging force is too low and this can lead to damage. If the contactengaging force is, however, greater than absolutely necessary to avoidslippage, then the efficiency of the transmission drops which leads toan unnecessarily high consumption of fuel. For this reason, it isadvantageous to so select the contact engaging force that the maximumtranmissible torque for this contact engaging force is only slightlygreater than the torque to be transmitted at the particular time. If ahigh slippage of the continuous element nonetheless occurs, thenmeasures must be taken in order to stabilize or stop the relativemovement of the continuous element so that damage is avoided.

SUMMARY OF THE INVENTION

Conventional continuously variable transmission controls, such asdisclosed in EP,A1,0 451 887, determine the transmission input torquefrom the torque, which is outputted by the engine, and the converteramplification. A belt tension is computed from this torque from whichbelt tension a contact engaging force results which makes possible areliable transmission of this torque. Here, as a rule, a considerablereserve of reliability is considered which causes a higher fuelconsumption. If an impermissibly high slippage nonetheless occurs, then,in general, the contact engaging force and therefore the belt tension isincreased. The speed with which a reaction can be had to excessiveslippage is, however, limited by the time constants of the hydraulic andof the mechanical system of the belt tensioning adjustment.

A system is known from EP,B1,0 446 497 (corresponding to US 5,098,345)for protecting against exceeding a maximum slippage of the belt. Here, aclutch is so controlled that this clutch always starts slipping at lowerdrive torques than the band. A reaction to a possible slippage of theband is not described here.

The detection of belt slippage is known in many configurations. Thus,and for example in accordance with U.S. Pat. No. 5,871,411, it issuggested to detect the speed of the continuous element. Furthermore,the belt slippage can be detected by a sensor which simulatneouslydetects the axial displacement of a conical-pulley pair and the rpm ofthis pair. From earlier German patent application 196 38 277.7, it isknown to provide at least two sensor units for slippage detection whichare mounted in the region of the continuous means and between the driveend and the output end.

The object of the present invention is to provide effective measures forexcessive slippage in order to reliably avoid damage to thetransmission.

As mentioned, the invention proceeds from a control of a transmission,which is continuously adjustable with respect to its transmission ratio,for a motor vehicle. The transmission is mounted in the drive train ofthe motor vehicle together with a drive unit having an adjustable drivetorque. The transmission has a drive end and an output end and operativemeans for establishing a mechanical operative connection between thedrive end and the output end. Furthermore, detecting means for detectinga slippage quantity is provided which represents the slippage betweenthe operative means and the drive end and/or output end. Pregivablemeasures are initiated in response to a pregivable value of the slipquantity. The essence of the invention is seen in that the following areprovided as measures:

a control of a clutch mounted in the drive train; and/or,

a change of the transmission ratio; and/or,

a change of the output torque of the drive unit.

It is especially intended here that the measures are initiated in thesense of avoiding slip.

In an advantageous embodiment of the invention, a further measure is achange of the contact engaging force between the drive end and/or theoutput end and the operative means, especially in the sense of avoidingslip.

The present invention describes how through slipping of the continuouselement can be stopped and stabilized to a tolerable slip with three orfour possible interventions when through slip of the continuous elementis detected. The invention offers the possibility to undertake theintroduced measures individually or with a coordinated strategy.Depending upon the particular situation, which led to a through slip ofthe continuous element, a response is provided in accordance with apreviously determined strategy having a selection of the measuresaccording to the invention simultaneously or in a tight time-dependentsequence. This always takes place with the objective to suppress adetected slip as quickly as possible and not to confuse the driver witheffects of the measures. Depending upon the equipped variation of thevehicle, it is possible that not all of these measures can be carriedout.

It is especially provided that:

the drive of the clutch takes place in the sense of an opening of theclutch; and/or,

the change of the transmission ratio takes place in response to apositive slip quantity in the sense of an increase of the transmissionratio and in response to a negative slip quantity in the sense of areduction of the transmission ratio; and/or,

the change of the output torque of the drive unit takes placein responseto a positive slip quantity in the sense of a reduction of the outputtorque and in response to a negative slip quantity in the sense of anincrease of the output torque.

Furthermore, it is especially advantageous that the following isdependent upon the extent of the detected slip quantity:

the extent of the drive of the clutch; and/or,

the extent of the change of the transmission ratio; and/or,

the extent of the change of the output torque of the drive unit; and/or,

the extent of the change of the contact engaging force.

In a further advantageous embodiment of the invention it is providedthat, simultaneously or in time after the drive of the clutch in thesense of an opening of the clutch, the following is provided: a changeof the transmission ratio; and/or, a change of the instantaneous outputtorque of the drive unit; and/or, a change of the contact engaging forcein such a manner that the maximum transmissible torque by thetransmission becomes again greater in magnitude than the magnitude ofthe instantaneous output torque of the drive unit. In this way, theclutch slip can again be reduced by an increase of the maximumtransmissible torque of the clutch (closing of the clutch).

It is especially provided that the change of the output torque of thedrive unit for slip reduction is actuated as support to at least one ofthe other measures (clutch drive and/or change of the transmission ratioand/or contact engaging force).

As already described, the transmission can be a continuous transmission.The drive end and/or the output end can have at least an axiallydisplaceable element which has essentially the form of a conical pulley.As operative means, at least a belt (preferably a thrust element belt)or a belt or a chain is tensioned between pulley pairs which define thedrive end and the output end.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows schematically a continuously variable transmission having aknown control of the contact engaging force; whereas,

FIG. 2 shows an embodiment with reference to a block circuit diagram.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1, a known configuration of a continuously variable transmissionis shown in section. The internal combustion engine 1 can be influencedby the throttle flap 2 with respect to its outputted torque M_(m). Thethrottle flap 2 is, for example, coupled mechanically or electrically tothe accelerator pedal (not shown). The engine 1 is coupled mostly bymeans of a clutch and/or a converter 3 to the drive end (primary end) ofthe CVT transmission 4. The output end (secondary end) of the CVTtransmission 4 is connected via a downstream transmission (not shown) tothe wheels of the vehicle. The CVT transmission has an axiallydisplaceable conical pulley on each of the primary and secondary ends.To adjust the transmission ratio, a corresponding primary pressure P_(p)or a secondary pressure P_(s) is built up in the oil chambers 7 and 8,respectively.

With a suitable selection of the actuating quantities of primarypressure P_(p) and secondary pressure P_(s) the following must beensured:

1. the transmission ratio i corresponds to the desired ratio of primaryrpm N_(p) and secondary rpm N_(s); and,

2. the force transmitting thrust element belt 9 (for example, chain,belt) is pressed against the pulleys with sufficient force in order toprevent a through slippage of the thrust element 9.

The above-mentioned point 1 is realized via an electro-hydrayulictransmission ratio or primary rpm control 10. For point 2, a belttension control 11 is used.

Rpm sensors 12, 13, and 14 are provided on the engine 1 and on the CVTtransmission 4 for the transmission ratio and belt tension control. Therpm sensors 12, 13 and 14 detect the engine rpm N_(m), the primary rpmN_(p), and the secondary rpm N_(s).

The master-slave principle is shown in FIG. 1 and is mostly utilized. Inthis master-slave principle, the secondary pressure P_(s) serves toadjust the belt tension and the primary pressure P_(p) serves to adjustthe transmission ratio rpm or the primary rpm. For the alternativepartner principle, the belt tension control influences the primarypressure as well as the secondary pressure.

Generally, one can state that a position signal in the form of apressure quantity P_(s) is available for the belt tension control. Fromthe literature, several methods for controlling the belt tension areknown, which however all work in a similar manner.

In FIG. 1, a sensor is identified by reference numeral 18 which detectsthe speed V_(b) of the belt 9. In block 24, the actual belt slip S isdetermined from the belt speed V_(b), the primary rpm N_(p) and thesecondary rpm N_(s) as described in the initially-mentioned U.S. Pat.No. 5,871,411. It is here noted that the slip determination can also beobtained differently without departing from the concept of theinvention.

In FIG. 2, the four possible interventions for slip reduction are shown.With reference numerals 27, 28 and 29, the drive motor 27 with thecorresponding engine control 22, the clutch 28 with the correspondingclutch control 23 as well as the CVT transmission 29 with thealready-described slip detection 24, the belt tensioning control 25 andthe transmission ratio control 26 are shown.

The individual actions for reducing the through slip of the continuouselement are matched by the block 21 “slip intervention” in response to adetected excessive slip S of the continuous element in order to stop theslip or to stabilize the slip to an acceptable slip. In dependence uponthe particular situation which led to a through slip of the continuouselement 9, a response is provided in accordance with a previouslydetermined strategy with a selection of measures (here described)simultaneously or in a tight time sequence. These measures areundertaken with the object of suppressing a detected slip as quickly aspossible and not to confuse the driver by the effects of the measures.

Not all of these measures can be carried out depending upon theequipment vairation of the vehicle.

Increase of the Belt Tension (Block 25)

If the slip detection 21 detects an increased slip S, then the tensionof the continuous element 9 is increased in order to reduce the slip Sto a tolerable amount via an increased contact engaging force. Theincrease ΔF takes place independently of the sign of the detected slipS.

It is especially advantageous to select the quantity ΔF of theadditional tension of the continuous element 9 in dependence upon theamount of the detected slip S. Here, it must be considered that thecontact engaging force must be greatly increased because of thetransition from static friction to sliding friction between thecontinuous element and the pulley in order to reduce slip. It istherefore advantageous when the additional belt tension ΔF is notlinearly dependent upon the detected slip S. As already mentioned, thespeed with which a response can be made is limited by the time constantsof the hydraulic system and of the mechanical system.

Transmission Ratio Adjustment (Block 26)

If the slip detection 24 detects an increased slip S, then thetransmission ratio i of the continuous transmission is so adjusted thatthe geometric transmission ratio corresponds again to the rpmtransmission ratio.

For a known positive slip, this means that the slip interventionrequires a positive additional transmission ratio Δi, that is, a largertransmission ratio is required than without intervention. On the otherhand, for a detected negative slip S, a negative additional transmissionratio Δi is required which reduces the transmission ratio i of thetransmission.

It is especially advantageous to select the magnitude of the additionaltransmission ratio Δi in dependence upon the detected slip S.

This intervention for positive slip is especially suitable for thepresent-day conventional constructions of the CVTs. For positive slip,for which the engine torque M_(m) can no longer be transmitted, theengine rpm increases greatly. The additional transmission ratio Δi canbe realized by releasing oil from the primary pulley without anadditional pumping capacity for hydraulically increasing the contactengaging force. This measure should, however, be combined with areducing engine torque intervention (block 22) which is yet to bedescribed.

Torque Intervention at the Engine (Block 22, 27)

If the slip detection 21 detects an increased slip S, then the torqueM_(m). which is outputted by the engine 27, can be changed in order toadapt the torque, which is to be transmitted by the transmission, to themaximum transmissible torque M_(max) and to thereby reduce the slip to atolerable amount.

For positive slip (that is, when the transmission input rpm N_(p) isgreater than it should be in accordance with the geometric transmissionratio), the torque M_(m), which is outputted by the engine 27, must bereduced (the additional torque ΔM_(m) is negative). For this purpose,all interventions can be used which are applicable to a known outputslip control (ASR). These interventions include, for example, anignition angle intervention for a spark-ignition engine not havingelectronic engine power control (E-gas). For engines having electronicengine power control (E-gas or EDC), simply a reduced torque isrequired. In each case, the slip intervention requires a negativeadditional torque ΔM_(m).

If the drag torque of the engine 27 with respect to magnitude is greaterthan the torque M_(max), which is maximally transmissible by thetransmission, then the transmission input rpm N_(p) is less than itshould be in accordance with the geometric transmission ratio. In thiscase, a negative belt slippage is present and the drag torque of theengine 27 should be reduced. All interventions can serve for thispurpose which are used at the present time for a known engine dragtorque control (MSR). For engines having electronic engine power control(E-gas or EDC), simply a higher torque is required. This means that theslip intervention requires a positive additional torque ΔM_(m) for anegative slip.

It is especially advantageous in both cases to select the magnitude ofthe additional torque ΔM_(m) at the engine 27 in dependence upon theamount S of the detected slip. These interventions are characterized bytheir relatively short time constants and should therefore always beapplied supportive to the other interventions presented herein.

Clutch Intervention (Block 23, 28)

If the vehicle has, for example, an electronically influenceable clutch28, then, when slip S of the continuous element 9 in the transmission isdetected, the maximum transmissible torque M_(K,max) of the clutch 28can also be reduced so that the slip occurs at the clutch 28 in lieu ofat the continuous element 9. This is advantageous because the clutch 28is so configured in its construction that is can withstand a higher slipfor a certain time without damage.

If the maximum transmissible torque M_(K,max) is reduced at the clutch28 by a clutch intervention ΔM_(K), then the torque at the transmissioninput cannot be greater in magnitude than the maximum transmissibletorque M_(K,max) of the clutch. In this way, it is possible to make thetorque of the transmission 29, which is to be transmitted, less inmagnitude than the maximum transmissible torque M_(get,max) of thetransmission 29 and to so limit the slip at the transmission to anoncritical value.

As long as the torque M_(m) at the engine output, the contact engagingof the continuous element 9 and the transmission ratio i of thetransmission do not change, the slip then occurs at the clutch 28instead of at the transmission 29. This is advantageous for theabove-mentioned reasons.

It is especially advantageous to ensure, via one of the other threedescribed measures, that the torque M_(get,max) which is the maximumtorque transmissible by the transmission 29, becomes greater again inmagnitude than the magnitude of the engine output torque in order to beable to reduce the slip at the clutch 28 via an increase (closure of theclutch) of the maximum transmissible torque M_(K,max) of the clutch.

In summary, it can be stated that the described possibilities can beapplied individually or in combination. Especially, in dependence uponthe situation, combined interventions should always be carried out sothat the driver does not notice the interventions on the slip control.Within a few milliseconds (50 to 590 ms), the condition wanted by thedriver should again be applicable in the drive train of the vehicle sothat the vehicle does not unexpectedly accelerate or decelerate.

What is claimed is:
 1. An arrangement for controlling a transmission fora motor vehicle having a drive unit and a drive train, the motor vehicleincluding a clutch disposed between the drive unit and the transmission,the transmission having a transmission ratio which is continuouslyvariable, the transmission and the drive unit being mounted in the drivetrain, the drive unit having an adjustable output torque (M_(m)), thetransmission having a drive end and an output end and the transmissionincluding operative means for providing a mechanical operativeconnection between the drive end and the output end thereof, thearrangement comprising: detection means for detecting a slip (S)representing the slip between said operative means and at least one ofsaid drive end and said output end of said transmission; means forinitiating a change (Δi) of the transmission ratio (i) as a firstmeasure in response to a pregiven value of said slip (S); means forinitiating at least one of the following measures in response to saidpregiven value of said slip (S): a change (ΔF) of said contact engagingforce between said drive end and/or said output end and said operativemeans in a direction to reduce said slip (S); a drive (ΔM_(K)) of saidclutch arranged in said drive train; and, means for initiating a change(ΔM_(m)) of said output torque of said drive unit as an additionalmeasure in response to said pregiven value of said slip (S).
 2. Anarrangement for controlling a transmission for a motor vehicle having adrive unit and a drive train, the motor vehicle including a clutchdisposed between the drive unit and the transmission, the transmissionhaving a transmission ratio which is continuously variable, thetransmission and the drive unit being mounted in the drive train, thedrive unit having an adjustable outgut torque (M_(m)), the transmissionhaving a drive end and an output end and the transmission includingoperative means for providing a mechanical operative connection betweenthe drive end and the output end thereof, the arrangement comprising:detection means for detecting a slip (S) representing the slip betweensaid operative means and at least one of said drive end and said outputend of said transmission; means for initiating a chance (Δi) of thetransmission ratio (i) as a first measure in response to a pregivenvalue of said slit (S); means for initiating at least one of thefollowing measures in response to said pregiven value of said slip (S):a change (ΔF) of said contact engaging force between said drive endand/or said output end and said operative means in a direction to reducesaid slip (S); a drive (ΔM_(K)) of said clutch arranged in said drivetrain; means for initiating at least one of the following measures inresponse to said pregiven value of said slip (S): causing the drive(ΔM_(K)) of said clutch to take place in the sense of an opening of theclutch; causing the change (Δi) of the transmission ratio (i) to takeplace in response to a positive slip quantity in the sense of anincrease in the transmission ratio (i) and in response to a negativeslip quantity in the sense of a reduction of the transmission ratio;and, causing the change (ΔM_(m)) of the output torque of the drive unitto take place in. response to a positive slip quantity in the sense of areduction of the output torque in response to a negative slip quantityin the sense of an increase of the output torque.
 3. An arrangement forcontrolling a transmission for a motor vehicle having a drive unit and adrive train, the motor vehicle including a clutch disposed between thedrive unit and the transmission, the transmission having a transmissionratio which is continuously variable, the transmission and the driveunit being mounted in the drive train, the drive unit having anadjustable output torque (M_(m)), the transmission having a drive endand an output end and the transmission including operative means forproviding a mechanical operative connection between the drive end andthe output end thereof, the arrangement comprising: detection means fordetecting a slip (S) representing the slip between said operative meansand at least one of said drive end and said output end of saidtransmission; means for initiating a change (Δi) of the transmissionratio (i) as a first measure in response to a pregiven value of saidslip (S); means for initiating at least one of the following measures inresponse to said pregiven value of said slip (S): a change (ΔF) of saidcontact engaging force between said drive end and/or said output end andsaid operative means in a direction to reduce said slip (S); a drive(ΔM_(K)) of said clutch arranged in said drive train; and, thetransmission has a maximum transmissible torque (M_(get,max)) and,simultaneously or in time after the drive (Δ_(K,max)) of the clutch inthe sense of an opening of the clutch, a change (Δi) of the transmissionratio and/or a change (ΔM_(m)) of the instantaneous output torque of thedrive unit and/or, a change of the contact engaging force (ΔF) is madein such manner that the torque (M_(get,max)) which is transmittedmaximally by the transmission, becomes greater than the magnitude(M_(m)) of the instantaneous output torque of the drive unit.
 4. Anarrangement for controlling a transmission for a motor vehicle having adrive unit and a drive train, the motor vehicle including a clutchdisposed between the drive unit and the transmission, the transmissionhaving a transmission ratio which is continuously variable, thetransmission and the drive unit being mounted in the drive train, thedrive unit having an adjustable output torque (M_(m)), the transmissionhaving a drive end and an output end and the transmission includingoperative means for providing a mechanical operative connection betweenthe drive end and the output end thereof the arrangement comprising:detection means for detecting a slip (S) representing the slip betweensaid operative means and at least one of said drive end and said outputend of said transmission; means for initiating a change (Δi) of thetransmission ratio (i) as a first measure in response to a pregivenvalue of said slip (S); means for initiating at least one of thefollowing measures in response to said pregiven value of said slip (S):a change (ΔF) of said contact engaging force between said drive endand/or said output end and said operative means in a direction to reducesaid slip (S); a drive (ΔM_(K)) of said clutch arranged in said drivetrain; wherein the transmission has a maximum transmissible torque(M_(get,max)); and, the clutch responds in the sense of a closure inresponse to the torque (M_(get,max)), which is the maximum torquetransmissible by the transmission and which becomes greater in magnitudethan the torque (M_(m)) of the instantaneous output torque of the driveunit.
 5. A method of controlling a transmission for a motor vehiclehaving a drive unit and a drive train, the motor vehicle including aclutch disposed between the drive unit and the transmission, thetransmission having a transmission ratio which is continuously variable,the transmission and the drive unit being mounted in the drive train,the drive unit having an adjustable output torque (M_(m)), thetransmission having a drive end and an output end and the transmissionincluding operative means for providing a mechanical operativeconnection between the drive end and the output end thereof, the methodcomprising the steps of: detecting a slip quantity (S) which representsthe slip between the operative means and at least one of the drive endand the output end of said transmission; initiating a chance (Δi) of thetransmission ratio (i) as a first measure in response to a pregivenvalue of said slip (S); and, initiating at least one of the followingmeasures in response to said pregiven value of said slip (S): a chance(6F) of said contact engaging force between said drive end and/or saidoutgut end and said operative means in a direction to reduce said slip(S); and, a drive (ΔM_(K)) of said clutch arranged in said drive train;wherein said measures include at least one of the following: causing thedrive (ΔM_(m)) of the clutch to take place in the sense of an opening ofthe clutch; causing the change (Δi) of the transmission ratio (i) inresponse to a positive slip quantity to take place in the sense of anincrease of the transmission ratio and, in response to a negative slipquantity, take place in the sense of a reduction of the transmissionratio; and/or, causing the change of the output torque (ΔM_(m)) of thedrive unit to take place in response to a positive slip quantity in thesense of a reduction of the output torque, in response to a negativeslip quantity in the sense of an increase of the output torque; and,wherein: the extent of the drive of the clutch; and/or, the extent ofthe change of the transmission ratio; and/or, the extent of the changeof the output torque of the drive unit is dependent upon the extent ofthe detected slip quantity and, as a further measure, a change of thecontact engaging force is provided between the drive and/or output endand the operative means, especially in the sense of a slip reduction.